1. Field of the Invention
The present invention relates to a solar heat collecting apparatus, and more particularly to an apparatus for collecting solar heat to produce a high-temperature medium.
2. Description of the Related Art
Solar heat collecting apparatuses are widely used as, for example, water heaters. In an ordinary solar heat collecting apparatus, a heat-absorbing member absorbs solar radiant energy and produces heat. An example heat-absorbing member would be a flat heat-absorbing plate. Commonly a pipe for conducting a medium is attached to such heat-absorbing plates, and the collected solar heat is then transmitted to the medium such as water in the pipe. In this way, high-temperature water can be produced.
However, in conventional apparatuses provided with such a heat-absorbing plate, heat is also transferred from the heat-absorbing plate to the ambient air by thermal conduction. As the air flows convectively, a great heat loss can occur through thermal conduction. A large amount of infrared radiation is emitted from the heat-absorbing plate to the surroundings, which results in additional considerable heat loss. Because of such heat losses, such as thermal conduction and heat radiation, in conventional apparatuses, it has been difficult to obtain water at higher than about 100.degree. C.
Heat absorption and heat loss of the heat-absorbing plate will be discussed in more detail.
The heat loss by infrared radiation from the heat-absorbing plate becomes greater as the temperature of the heat-absorbing plate rises. For a typical 1 m.sup.2 heat-absorbing plate with an emissivity of 0.9 at 150.degree. C. and a gray radiator with an emissivity of 0.9 at 30.degree. C. provided surrounding the heat-absorbing plate, the energy radiated from the heat-absorbing plate to the gray radiator amounts to as much as 1 kW/m.sup.2 on only one surface of the heat-absorbing plate, and therefore a total radiation loss from both surfaces of the above-mentioned heat-absorbing plate is no less than 2 kW/m.sup.2. On the other hand, the solar radiant energy absorbed by the above-mentioned heat-absorbing plate can be estimated at about 800 kW/m.sup.2. With all of these figures taken into account, it is not possible to produce water with a temperature more than about 80-90.degree. C.
As is obvious from the foregoing, in such conventional apparatuses, it has been difficult to decrease heat loss by effectively suppressing the heat radiation and thermal conduction from the heat-absorbing plate. It is desired that the utilization ratio of solar heat should be improved.